Earthquake "Swarm" Strikes Africa, Puzzles Experts

Nicholas Wadhams in Nairobi, Kenya
for National Geographic News
July 24, 2007

The tremors began on July 12, so faint that they were barely noticed. A week later, a couple of good jolts sent people fleeing their offices in downtown Nairobi. Then the tremors were gone.

Scientists are still unsure about what exactly caused this "earthquake swarm"—a cluster of relatively mild shakes spaced out over several days in Kenya and Tanzania. The quakes—one of which reached magnitude 5.9—caused little damage but spread fear that a big quake was imminent. (What is an earthquake?)

Was it fault activity along the Rift Valley, where the African tectonic plate is stretching? Or was it Ol Doinyo Lengai, a Tanzanian volcano that sits near the swarm's epicenter?

"Mountain of God" Takes the Heat

So far, government officials have pushed the theory that rumbling in Ol Doinyo Lengai—whose name means "Mountain of God" in Masai, was to blame for the tremors.

Fears that a massive eruption was near triggered several hundred people to evacuate its slopes, and the Tanzanian government warned tourists to stay away.

Kenyan government spokesman Alfred Mutua reported that the 9,470-feet (2,886-meters) tall volcano had erupted Friday and that the eruption had eased the pressure that caused the swarm of at least a dozen mini-quakes.

Scientists at Dar es Salaam University in Tanzania said there was indeed evidence of some sort of eruption at the volcano and that they would climb the mountain to investigate.

The mountain has a history of playing tricks on people. Tremors can loosen rocks that create dust as they tumble down the side, resembling lava flows leaving a trail of ash and steam down the mountain.

In March 2006 such a scare caused an evacuation, when smoke from a grass fire on the slope created the appearance of an eruption, said Celia Nyamweru, an anthropology professor at St. Lawrence University in New York.

Rift Valley Rumblings

Another possible cause of the tremors, scientists say, is that the earthquake swarm had nothing to do with the volcano at all, but was the result of tectonic activity in Africa's Great Rift Valley, which stretches approximately 3,700 miles (6,000 kilometers) in length.

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